This invention relates in general to semiconductor power devices and in particular to a trench MOS-gated transistor with reduced miller capacitance.
FIG. 1 shows a simplified cross-section view of a conventional vertical trenched gate MOSFET 100. An epitaxial layer 104 of n-type conductivity type extends over n-type substrate 102 which forms the drain contact region. Well region 106 of p-type conductivity type is formed in an upper portion of epitaxial layer 104. Gate trench 109 extends through well region 106 and terminates just below the interface between epitaxial layer 104 and well region 106. Gate trench 109 is lined with a dielectric layer 112 along its sidewalls and bottom, and is filled with polysilicon material 110 forming the transistor gate. Source regions 108 flank each side of trench 109, and overlap gate 110 along the vertical dimension. In the on-state, a current flows vertically from drain terminal 114 to source terminal 116 through substrate 102, epitaxial layer 104, channel regions in well region 106 along the outer sidewalls of trench 109, and finally source regions 108.
Epitaxial layer 104 together with substrate 102 form the drain region. As can be seen, gate 110 overlaps the drain region along the bottom of trench 109. It is desirable to minimize this gate-drain overlap in order to improve the transistor switching speed. The gate-drain charge Qgd is proportional to this overlap area and inversely proportional to the thickness of the dielectric along the bottom of trench 109. Several methods to reduce Qgd have been proposed including reducing the trench width, using thicker dielectric along the trench bottom, eliminating portions of the gate along the trench flat bottom portion, and extending the p-type well region slightly deeper than the trench. Each of these techniques has its own advantages and disadvantages. Some require a more complex process technology, while others are not as effective in reducing Qgd without adversely impacting other device characteristics.
Thus, an MOS-gated transistor with improved characteristics including a substantially reduced miller capacitance, and which is simple to manufacture is desirable.